The present invention relates to a hollow blow-molded container of a biaxially oriented polyester resin and, more specifically, to a process for manufacturing such a container of hollow bottle shape free from crazing regardless of the type of its contents, in which such sections as neck, neck end or bottom section thereof where the resin is not substantially subjected to orientation are protected against thermal deformation.
Polyethylene terephthalate has a wide range of applications in the field of containers for foodstuffs, flavoring materials, cosmetics and so on, because it can be molded, by orientation-blowing, into transparent thin-walled containers having a high stiffness, impact strength and improved hygienic qualities with a higher molding accuracy.
In ordinary direct blow molding processes in which the top and bottom parts of an extruded parison are held by a mold and compressed air is blown into the thus held parison to expand the same into a container shape, resultant containers may often be unsatisfactory in respect of strength and transparency because the parison is oriented only monoaxially. Therefore, in blow molding, there prevails a so-called biaxially-orienting blow-molding process in which the parison is oriented not only laterally but also longitudinally in a temperature range suitable for such orientation, and biaxially-oriented blow-molded containers show increased stiffness and strength as well as improved gas barrier properties and transparency. However, even in such a biaxially-orienting blow-molding process, such sections as the neck, neck end and bottom section of the resultant containers cannot enjoy an improvement in physical properties and are susceptible to thermal deformation, because such sections are not subjected to orientation, that is to say, molecular orientation never or hardly occurs and the bond between the molecular chains constituting polymer crystals is not strong in these sections.
As is well-known, those containers which are used for storing volatile liquids require very tight sealing. Therefore, the neck ends of such containers are usually sealed with a crown cap caulked thereon or a cap placed thereon with interposition of a packing. However, if the neck section is deformed by some external factors such as heat, the aforementioned sealing effect of the sealing means is diminished. While, if the polyester containers are used to store highly-concentrated alcohol, ester, or cosmetics or solvents containing these, the aforementioned non-oriented sections may be permeated by the content to be crazed and, eventually, the liquid content may leak from the neck end. Also, if such containers are filled with carbonated beverages, their bottoms may be bulged out due to an increase in internal pressure and crazing to such an extent to debase their standing stability and, ultimately, the containers may burst. On the other hand, when such containers are used to store such liquids as juice or sauce that are filled therein under heat-sterilization, their neck end portions may be deformed, due to the filling temperature, to such an extent that the containers cannot be sealed completely even with a cap having an inner sealing seat.